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<br />a. A combination of streamflow and reservoir storage releases which <br />reflect the most adverse period of record which occurred in the <br />1928 to 1964 time frame to determine firm water supply for consump- <br />tive use purposes, and <br /> <br />b. The long-term hydrologic record beginning in 1906 to determine <br />power production and, in the future, to analyze salinity impacts. <br /> <br />3. Rather than a single projection of mean salinity at future points in <br />time, the results of a set of historical hydrologic events provide an <br />idea of the range of salinity that can be expected if the extremes of <br />history are reflected in the future. The resulting probability projec- <br />tions of expected salinity levels identify the most probable future <br />salinity levels and provide a view of the impacts of drought and flood <br />events on the salinity of the Colorado River. <br /> <br />Of the three analytic differences described above, the change in the <br />assumptions regarding the period of record used for analyses has the most <br />significant impact because of the larger average virgin flow that results. <br />The average Lee Ferry vi rgi n fl ows duri ng the 1941 through 1978 and 1906 <br />through 1974 periods are about 14 and 15 million acre-feet, respectively. <br />Although the salt loading also increases marginally, the resulting addi- <br />tional diluting water results in significantly lower future salinity <br />projections when the longer term hydrology is used. However, by using <br />the longer period of record, extremes as well as average conditions are <br />reflected in the resulting probabilities of occurrence, thereby providing a <br />more complete view of future salinity conditions. <br /> <br />It has been assumed for purposes of the "present modi fi ed method II study <br />that the average annual rate of pickup of di ssolved sol ids from newly <br />irrigated 1 ands woul d be in the range of 0 to 2 tons per acre per year. <br />This assumption is based upon previous detailed studies. Where comprehen- <br />sive studies showed a different rate, the different figure was used. The <br />effect of salt contributed from new lands is thus evaluated using these <br />rates. It was also assumed that no increase in the rate of pickup of <br />dissolved solids would occur for lands already under irrigation. Large <br />industries such as powerplants were considered to have no return of salts <br />or water, and salt load changes due to municipal and minor industries were <br />assumed negligible. <br /> <br />The Forum also made extensive salt routing studies for their 1981 review <br />using a computer model developed by Reclamation [6J. The salt routing <br />studies were conducted to provide estimates of future flow-weighted average <br />annual salinity concentrations for each year of the 1980 through 1995 study <br />period at selected points in the lower basin under differing assumptions as <br />to both the avail ab 1 e water supply and future water uses. Differences in <br />depletions and estimates of base salt load account for the different but <br />similar projections in the CRSS method and Forum method. The studies were <br />designed to provide estimates of sal inity concentrations both with and <br />without the implementation of salinity control projects. <br /> <br />34 <br />